Modern integrated circuits commonly come in the form of chips. The integrated circuits within a chip communicate with the world outside the chip through metalization layers on the outside of the chip known as signal pads. For communication from within the chip to the world outside, “I/O driver circuits” drive signals or data through the signal pads on the exterior of the chip. The signal pads of various chips are connected together by signal lines thereby allowing communications between different chips.
The electrical connection from the signal pad through the packaging of the chip and the load (on-board transmission line and the load itself) contains parasitic resistance, inductance, and capacitance which interferes with the transmission of the signal from the signal pad. The printed circuit board signal trace itself also contains transmission line characteristic impedance which also interfere with the quality of the transmitters of the signal from the signal pad.
To transmit a signal from one chip to another, it is necessary that the chip transmitting the signal have an appropriate signal driving capability, which drives an appropriate signal waveform for representing information contained in the signal. In general, a large driving capability means a small corresponding driving impedance, and vice versa. For best data transfer performance, the impedance of the I/O driver and the combined impedances of the on-board transmission line and the load should match.
Since the driving impedance is a crucial reference and parameter for driving output signals, a tolerable range allowed in the driving impedance is established in certain signal exchange interface standards. For example, in the Double-Data-Rate (DDR) interface standards, an impedance tolerable range for driving impedance is specified to maintain accuracy in signal exchange. Coupled with ever-increasing frequency expectations and power limitations, the tolerable driving range shrinks.